1. Field of the Invention
The present invention relates to a method for preparing by polymerization a polymer rod having a uniform composition in an axial direction thereof. More particularly, it relates to a method for preparing a polymer rod having a uniform composition in an axial direction by slowly feeding a monomer liquid to a vessel through the upper portion thereof, and then allowing polymerization reaction to proceed, while a part of the monomer is continuously volatilized from the surface of the monomer liquid.
Furthermore, the present invention also relates so a method for preparing a polymer rod, for example, a synthetic resin optical transmission medium having a continuously varied refractive index distribution in the cross section of the polymer rod. More specifically, it relates to a method for preparing a polymer rod as a synthetic resin optical transmission medium which has a refractive index distribution continuously varied in its cross section and a uniform distribution of composition in the direction of its length. It can be used as optical lenses and optical fibers. The polymerization reaction is carried out wish utilizing the difference in gel concentrations in the surface of a liquid mixture and the difference in the liability of monomers to volatilize, while a monomer liquid mixture is slowly fed to a vessel from the upper portion thereof.
2. Description of the Prior Art
Optical transmission media such as lenses and optical fibers are classified into two kinds of types, a single mode type and a multimode type in view of the mode of light transmission.
Among the multimode type optical transmission media, the graded index type optical transmission media having a refractive index distribution varying continuously in a fixed direction are widely used as bar lenses having the function of convex lenses, bar lenses having the function of concave lenses, and broad band optical transmission fibers. Among them, the optical transmission media made of transparent synthetic resins are more widely used in recent years, because they have several advantages in view of light weight, economy, easy handling, high impact resistance and flexibility as compared with those made of quartz.
The optical transmission medium made of a synthetic resin is usually used as a bar lens or an optical transmission fiber which comprises a polymer rod obtained by polymerization or by polymerization and then drawing.
Concerning the preparation of a refractive index distribution type optical transmission medium from a synthetic resin by polymerization reaction in a specific reaction vessel, the following methods have heretofore been proposed.
In U.S. Pat. No. 3,955,015, a monomer to form a polymer having a different refractive index is subjected to diffusion transfer into a previously formed specific transparent solid substance in a partially polymerized state having a three-dimensional reticular structure. After that, the whole polymerization reaction is stopped to obtain a rod-like optical transmission medium of a refractive index distribution type.
In this method, however, it is necessary that the transparent solid substance is previously made into the three-dimensional reticular structure by the use of a multi-functional radically polymerizable monomer in order to maintain the configuration of the transparent solid substance. For this reason, it must previously be made separately, which costs much labor. In addition, the obtained polymer having the three-dimensional reticular structure is not good in thermoplasticity and it is not suitable for other post-forming processes such as drawing. In other words, the practical plastic optical transmission fibers should have stiffness and tensile strength as fiber materials in the drawing step in the manufacturing process. However, the optical transmission medium prepared through the above process has the inherently three-dimensional reticular structure, so that it is not suitable for drawing operation.
In Japanese Laid-Open Patent Publication No. 61-130904, much attention is paid to a difference between the monomer reactivity ratios of two kinds of monomers, and by the utilization of this, there is proposed a method for producing an optical transmission medium having a refractive index gradient. However, in the medium obtained by this method, a good refractive index distribution is not always obtained, and cavities are easily formed owing to volume contraction attributed to polymerization. Thus, it is difficult to obtain an entirely uniform and large sized polymer suitable for industrial production. Furthermore, in the case that an acrylic resin transparent polymerization tube is used, the acrylic resin is dissolved in the monomer at a non-polymerizing position such as the position of the polymerization tube which is immersed in the monomer and to which radical generation energy is not applied, or the position of the polymerization tube which comes in contact with a monomer vapor, so that the thickness of the polymerization tube is uneven and the uniform polymer rod cannot be obtained. In an extreme case, the pipe is dissolved, and consequently it is torn to pieces or made full of holes.
In Japanese Laid-Open Patent Publication No. 61-170705, a method for successively feeding a monomer liquid mixture to a long molding tube is described, but even in this method, a good refractive index distribution cannot always be obtained. When a synthetic resin molded tube is used, the resin portion above the surface of the liquid mixture is dissolved by a monomer vapor, so that the wall thickness of the resin tube is reduced, and in an extreme case, the resin tube is dissolved off. Thus, a polymer uniform in the vertical direction cannot be formed.
The phenomena described above occur even when only one kind of monomer is used.
The present inventors have carried out extensive investigations concerning the process to form a copolymer resin from a liquid mixture of different radically polymerizable monomers. That is, when the viscosity of the monomer liquid mixture rises and the liquid turns into a gel with the progress of the polymerization of the monomers, the growing polymer radical is hardly diffused in the gel because the molecular weight of the polymer radical is large. In this case, the termination reaction between two molecules of the growing polymer radicals hardly proceeds, and as a result, a polymerization rate increases.
The above phenomenon is accepted as the so-called gel effect in the radical polymerization. When the radical polymerization is carried out so that the gel effect is produced from any position in a reaction vessel, the polymerization gradually proceeds toward an opposite end portion.
The inventors of the present application have made further investigation concerning a process in which, for example, two kinds of different monomers are bonded to propagating polymer radicals in a gel. That is, when the polymerization is allowed to proceed from a wall portion to a central portion of a cylindrical vessel, a gel having a higher polymer content is formed on the vessel wall portion in the vicinity of the liquid mixture surface, and the polymer content decreases toward the central portion of the liquid mixture surface. In general, there is a tendency that when a polymer solution containing a volatile substance has a high polymer content, the volatile substance is difficult to volatilize from the solution surface. That is, in the vicinity of the vessel wall where the polymer content is high, the amount of the volatilized substance is smaller than in the central portion of the vessel where the polymer content is low. Usually, when monomers are volatilized from the surface of a solution comprising two kinds of polymerizable monomers, the monomer having a lower boiling point volatilizes more largely, irrespective of a concentration ratio of the monomers constituting the solution. Therefore, in the vicinity of the vessel wall where the amount of the volatilized substance per unit time is small, a difference between the amount of the decreased monomer having a lower boiling point and that of the decreased monomer having a higher boiling point is smaller than in the central portion where the amount of the volatilized substance per unit time is large. In consequence, in the case that, of the selected two kinds of monomers, the monomer having a lower boiling point has a lower refractive index, a convex distribution is formed in which the refractive index increases from the wall portion to the central portion of the vessel. On the other hand, in the case that the monomer having the lower boiling point has a higher refractive index, a concave distribution is conversely formed.
Furthermore, for example, in the case that the vessel wall is composed of a monomer-soluble polymer, a gel containing the dissolved vessel wall polymer tends to settle by gravity. However, when the settlement occurs, the obtained polymer rod is liable to have an uneven structure. In order to prevent the settlement, it is necessary that in the lower portion of the gel containing the dissolved vessel wall polymer, polymerization substantially completes and the unreacted monomer is not present or a little present.
Experiments were done by the present inventors. That is, a monomer was slowly fed to a vessel through the upper portion thereof, and adjustment was made so that polymerization may proceed as much as the amount of the fed monomer, whereby the polymerization was allowed to proceed in compliance with a monomer feed rate. As a result, the volume of the unreacted monomer portion was maintained at a constant level to prevent the gel from settling, which permits uniformly growing the polymer in a vertical upward direction. That is, it has been found that when the fresh monomers are always slowly fed only in the vicinity of the liquid surface in the vessel, a refractive index distribution is formed by the utilization of a difference between ratios of the volatilized monomers and a difference between amounts of the volatilized monomers, and as a result, the polymer grows in a vertical upward direction in accordance with a monomer feed rate to obtain the polymer having a refractive index gradient continuously uniformly extending in the vertical upward direction and in a horizontal direction.
In the disclosure in Japanese Laid-Open Patent Publication No. 61-170705, polymerization is allowed to proceed from a lower portion of a long molded pipe without a forced exhaust means. Therefore, the monomer does not volatilize, so that the polymer rod having a good refractive index distribution cannot always be obtained.